Multi-Soil-Layering, the Emerging Technology for Wastewater Treatment: Review, Bibliometric Analysis, and Future Directions

Round 1

Reviewer 1 Report

1) Add nomenclature to the manuscript,

2) The multi-soil-layering (MSL) system is considered an emerging ecotechnology that has proved its efficiency in the treatment of a wide range of wastewater across the world. What about the disadvantages and limitations of such technology?

3) For Table 2. Modeling approaches used for water quality indicators in the MSL system, add the conditions and the limitations of the use of the developed models

4) The abstract and the conclusion should be more quantitative.

5) All References should be checked and presented correctly, as an example reference 64 (Nguyen, X.C.; Chang, S.W.; Tran, T.C.P.; Nguyen, T.T.N.; Hoang, T.Q.; Banu, J.R.; Al-Muhtaseb, A.H.; La, D.D.; Guo, W.; Ngo, H.H.; et al. Comparative Study about the Performance of Three Types of Modified Natural Treatment Systems for Rice Noodle Wastewater. Bioresour. Technol. 2019, 282, 163–170) contains #et al#, please write the names in the place of #et al#

6) This article needs to be formatted carefully and the language should be re-approved by a native speaker/Grammarly checked.

Author Response

Reviewer 1 :

• Add nomenclature to the manuscript,

According to the reviewer's suggestion, we have added a nomenclature to the manuscript as indicated on page 1-2, line 34-72 as follows:

Nomenclature

Al: aluminium

BCS: blended organic carbon source

BOD₅: five-day biochemical oxygen demand

CH₄: methane

CO₂: carbon dioxide

COD: chemical oxygen demand

CW: constructed wetland

DO: dissolved oxygen

EC: electrical conductivity

FC: fecal coliform

Fe⁰: iron

Fe²⁺: ferrous ion

Fe³⁺: ferric ions

FIB: fecal bacteria indicator

HLR: hydraulic loading rate

MLR: multiple linear regression

MSL: multi-soil-layering

N₂: diazote

N₂O: nitrous oxide

NH₃: ammonia

NH₄⁺: ammonium

NN: neural networks

NO₃^–: nitrates

NO₂^–: nitrites

ORP: oxidation-reduction potential

PBS: polybutylene succinate

PL: permeable layer

Q: salinity

SCA: stepwise-cluster analysis

SFCW: subsurface flow constructed wetland

SS: suspended solids

SWI: subsurface wastewater infiltration

TC: total coliform

TN: total nitrogen

TKN: total kjeldahl nitrogen

TP: total phosphorus

VFCW: vertical flow constructed wetland

WOS: web of sciences

• The multi-soil-layering (MSL) system is considered an emerging ecotechnology that has proved its efficiency in the treatment of a wide range of wastewater across the world. What about the disadvantages and limitations of such technology?

The following information was added in table 2, on line 706 in response to the reviewer's query on the MSL system's limitations:

12. An, C.J.; McBean, E.; Huang, G.H.; Yao, Y.; Zhang, P.; Chen, X.J.; Li, Y.P. Multi-Soil-Layering Systems for Wastewater Treatment in Small and Remote Communities. J. Environ. Informatics 2016, 27, 131–144.
13. Zidan, K.; Sbahi, S.; Hejjaj, A.; Ouazzani, N.; Assabbane, A.; Mandi, L. Removal of Bacterial Indicators in On-Site Two-Stage Multi-Soil-Layering Plant under Arid Climate (Morocco): Prediction of Total Coliform Content Using K-Nearest Neighbor Algorithm. Environ. Sci. Pollut. Res. 2022, 29, 75716 - 75729.

• For Table 2. Modeling approaches used for water quality indicators in the MSL system, add the conditions and the limitations of the use of the developed models

In Table 3 (formerly Table 2), page 24, line 769, we have included two columns that display the conditions and limitations of these models as suggested by the reviewer.

Note: NN = Neural network; SCA = Stepwise cluster analysis; MLR = Multiple linear regression; QPF = Quadratic polynomial function

84. Kuhn, M.; Johnson, K. Applied Predictive Modeling, 1st ed.; Springer-Verlag: New York, USA, 2013; pp. 615.
85. Wang, X.; Huang, G.; Lin, Q.; Nie, X.; Cheng, G.; Fan, Y.; Li, Z.; Yao, Y.; Suo, M. A Stepwise Cluster Analysis Approach for Downscaled Climate Projection–A Canadian Case Study. Environ. Model. Softw. 2013, 49, 141–151.
86. Zhuang, X.W.; Li, Y.P.; Huang, G.H.; Liu, J. Assessment of Climate Change Impacts on Watershed in Cold-Arid Region: An Integrated Multi-GCM-Based Stochastic Weather Generator and Stepwise Cluster Analysis Method. Clim. Dyn. 2016, 47, 191–209.
87. Elmaz, F.; Yücel, Ö.; Mutlu, A.Y. Predictive Modeling of Biomass Gasification with Machine Learning-Based Regression Methods. Energy. 2020, 191, 116541.
88. Guthery, F.S.; Bingham, R.L. A Primer on Interpreting Regression Models. J. Wildl. Manage. 2007, 71, 684–692.
89. Shrestha, N. Detecting Multicollinearity in Regression Analysis. Am. J. Appl. Math. Stat. 2020, 8, 39–42.
90. Wang, Y.M.; Elhag, T.M.S. A Comparison of Neural Network, Evidential Reasoning and Multiple Regression Analysis in Modelling Bridge Risks. Expert Syst. Appl. 2007, 32, 336–348.
91. Zhou, J.; Li, E.; Wei, H.; Li, C.; Qiao, Q.; Armaghani, D.J. Random Forests and Cubist Algorithms for Predicting Shear Strengths of Rockfill Materials. Appl. Sci. 2019, 9, 1621.
92. Zhang, Q.; Wu, Z.; Guo, G.; Zhang, H.; Tarolli, P. Explicit the Urban Waterlogging Spatial Variation and Its Driving Factors: The Stepwise Cluster Analysis Model and Hierarchical Partitioning Analysis Approach. Sci. Total Environ. 2021, 763, 143041.
93. Liu, Y.R.; Li, Y.P.; Sun, J. Statistical Downscaling of Temperature Using Stepwise Cluster Analysis Method–a Case Study in Nur Sultan, Kazakhstan. IOP Conference Series: Earth and Environmental Science. 2020, 435, 12019.
94. Sun, W.; Shi, Q.; Huang, Y.; Lv, Y. Ensemble Learning Enhanced Stepwise Cluster Analysis for River Ice Breakup Date Forecasting. J. Environ. Inf. Lett. 2019, 1, 37–47.
95. Huang, G.H.; Huang, Y.F.; Wang, G.Q.; Xiao, H.N. Development of a Forecasting System for Supporting Remediation Design and Process Control Based on NAPL‐biodegradation Simulation and Stepwise‐cluster Analysis. Water Resour. Res. 2006, 42, 1-19.
96. Shokrzadeh, S.; Jozani, M.J.; Bibeau, E. Wind Turbine Power Curve Modeling Using Advanced Parametric and Nonparametric Methods. IEEE Trans. Sustain. Energy. 2014, 5, 1262–1269.
97. Békés, G.; Kézdi, G. Data Analysis for Business, Economics, and Policy; Cambridge University Press: Cambridge, United Kingdom, 2021; pp. 738.
98. Sahoo, S.; Jha, M.K. Groundwater-Level Prediction Using Multiple Linear Regression and Artificial Neural Network Techniques: A Comparative Assessment. Hydrogeol. J. 2013, 21, 1865–1887.
99. Strutz, J.; Martin, J.; Greene, J.; Broadbelt, L.; Tyo, K. Metabolic Kinetic Modeling Provides Insight into Complex Biological Questions, but Hurdles Remain. Curr. Opin. Biotechnol. 2019, 59, 24–30.
100. Srinivasan, S.; Cluett, W.R.; Mahadevan, R. Constructing Kinetic Models of Metabolism at Genome‐scales: A Review. Biotechnol. J. 2015, 10, 1345–1359.

• The abstract and the conclusion should be more quantitative.

We have updated the abstract by including quantitative data as recommended by the reviewer as indicated below:

• All References should be checked and presented correctly, as an example reference 64 (Nguyen, X.C.; Chang, S.W.; Tran, T.C.P.; Nguyen, T.T.N.; Hoang, T.Q.; Banu, J.R.; Al-Muhtaseb, A.H.; La, D.D.; Guo, W.; Ngo, H.H.; et al. Comparative Study about the Performance of Three Types of Modified Natural Treatment Systems for Rice Noodle Wastewater. Bioresour. Technol. 2019, 282, 163–170) contains #et al#, please write the names in the place of #et al#

According to the reviewer's note, all references have been checked and updated in accordance with the water journal requirements.

• This article needs to be formatted carefully and the language should be re-approved by a native speaker/Grammarly checked.

According to the reviewer’s suggestion, the paper's grammar has been reviewed, its language has been re-verified as highlighted in the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

I reviewed the article with the title ``Multi-soil-layering, the emerging technology for wastewater treatment: Review, bibliometric analysis, and future directions ``.  The article topic is intriguing and promising in the area. Overall, the article structure and content are suitable for the Water journal. I am pleased to send you major-level comments, there are some flaws that need to be corrected before publication. Please consider these suggestions as listed below. 

·         The title is fine

·         The abstract seems to be fine. Please add one more introductory line of your objective at beginning of the abstract. Highlight the core idea.

·         Keywords are ok

·         Research gap should be delivered in a clearer way with the directed necessity for future research work.

·         Introduction section must be written in a more quality way, i.e., more up-to-date references addressed. Please target the specific gap.

·         The novelty of the work must be clearly addressed and discussed, compare previous research with existing research findings, and highlight novelty.

·         Page 1 Line 41 need a reference, cite this article from literature- Role of nanomaterials in the treatment of wastewater: a review.

·         Page 2 line 49 need a reference. Please cite this- Introduction of adsorption techniques for heavy metals remediation.  

·         The last paragraph of introduction seems very weird, please do not use long sentence.

·         What is the main challenge? Please highlight this in the introduction part.

·         The main objective of the work must be written in the clearer and more concise way at the end of the introduction section.

·         Please provide space between numbers and units. Please revise your paper accordingly since some issue occurs in several spots in the paper. 

·          Please check the abbreviations of words throughout the article. All should be consistent.

·         Please add a comparative profile section as it is a review.

·         To avoid grammar and linguistic mistakes, moderate level English language should be thoroughly checked. Please revise your paper accordingly since several language issues occur in several spots in the paper.

·         Reference formatting needs careful revision. All must be consistent in one formate. Please follow the journal guidelines.

Author Response

• The abstract seems to be fine. Please add one more introductory line of your objective at beginning of the abstract. Highlight the core idea.

As suggested by the reviewer, we have highlighted the main idea of the paper in the abstract section as indicated on page 1 lines 18-19 as follows:

This paper attempts to comprehensively evaluate the application of MSL technology and investigate its progress and efficacy since its emergence.

• Research gap should be delivered in a clearer way with the directed necessity for future research work.

Research gaps have been presented in a clearer manner with a focus on the need for more study, as advised by the reviewers. This information is updated on page 2-3 line 100-113 as follows:

The current work sheds light on a number of subjects that MSL's earlier research did not address. For instance, most of the studies are still locked in laboratories and at the experimental level [15–17], whereas its full-scale experiment still has a weakness [18]. Regarding MSL efficiency, although the removal of organic matter and phosphorus is very promising [19,20], total nitrogen removal efficiency has not yet achieved this level [13,21]. Moreover, despite using a two-stage MSL system with the vertical flow, the high removal rate of coliform bacteria recorded did not exceed 3.15 log units [7], requiring the development of additional methods to improve MSL sanitary efficiency. Similarly, there is a weakness in research on viruses, pathogens, emerging pollutants, and their removal mechanisms [14,22]. As for combinations, several hybrid configurations are still under experimentation [23–27]. However, in this literature review, it was observed that there is a lack of studies investigating two-stage MSL with vertical and horizontal flow [18]. Regarding modeling approaches, although their results were encouraging [6,10,24], the application of other powerful methods and hybrid models may help to investigate pollutant removal in the MSL system [28,29].

7. Latrach, L.; Ouazzani, N.; Hejjaj, A.; Mahi, M.; Masunaga, T.; Mandi, L. Two-Stage Vertical Flow Multi-Soil-Layering (MSL) Technology for Efficient Removal of Coliforms and Human Pathogens from Domestic Wastewater in Rural Areas under Arid Climate. Int. J. Hyg. Environ. Health. 2018, 221, 64–80.
8. Hong, Y.; Huang, G.; An, C.; Song, P.; Xin, X.; Chen, X.; Zhang, P.; Zhao, Y.; Zheng, R. Enhanced Nitrogen Removal in the Treatment of Rural Domestic Sewage Using Vertical-Flow Multi-Soil-Layering Systems: Experimental and Modeling Insights. J. Environ. Manage. 2019, 240, 273–284.
9. Zidan, K.; Sbahi, S.; Hejjaj, A.; Ouazzani, N.; Assabbane, A.; Mandi, L. Removal of Bacterial Indicators in On-Site Two-Stage Multi-Soil-Layering Plant under Arid Climate (Morocco): Prediction of Total Coliform Content Using K-Nearest Neighbor Algorithm. Environ. Sci. Pollut. Res. 2022, 29, 75716 - 75729.
10. Sato, K.; Iwashima, N.; Wakatsuki, T.; Masunaga, T. Quantitative Evaluation of Treatment Processes and Mechanisms of Organic Matter, Phosphorus, and Nitrogen Removal in a Multi-Soil-Layering System. Soil Sci. Plant Nutr. 2011, 57, 475–486.
11. Song, P.; Huang, G.; An, C.; Zhang, P.; Chen, X.; Ren, S. Performance Analysis and Life Cycle Greenhouse Gas Emission Assessment of an Integrated Gravitational-Flow Wastewater Treatment System for Rural Areas. Environ. Sci. Pollut. Res. 2019, 26, 25883–25897.
12. Li, D.; Wang, X.; Chi, L.; Zhang, Z.; Liu, Y.; Li, X. Decentralized Domestic Sewage Treatment Using an Integratedmulti-Soil-Layering and Subsurface Wastewater Infiltration System. Water. 2021, 13, 431.
13. Maeng, S.K.; Park, J.W.; Noh, J.H.; Won, S.Y.; Song, K.G. Dissolved Organic Matter Characteristics and Removal of Trace Organic Contaminants in a Multi-Soil-Layering System. J. Environ. Chem. Eng. 2021, 9, 105446.
14. Zhang, P.; Huang, G.; An, C.; Fu, H.; Gao, P.; Yao, Y.; Chen, X. An Integrated Gravity-Driven Ecological Bed for Wastewater Treatment in Subtropical Regions: Process Design, Performance Analysis, and Greenhouse Gas Emissions Assessment. J. Clean. Prod. 2019, 212, 1143–1153.
15. Nguyen, X.C.; Chang, S.W.; Tran, T.C.P.; Nguyen, T.T.N.; Hoang, T.Q.; Banu, J.R.; Al-Muhtaseb, A.H.; La, D.D.; Guo, W.; Ngo, H.H. Comparative Study about the Performance of Three Types of Modified Natural Treatment Systems for Rice Noodle Wastewater. Bioresour. Technol. 2019, 282, 163–170.
16. Koottatep, T.; Suksiri, P.; Pussayanavin, T.; Polprasert, C. Development of a Novel Multi-Soil Layer Constructed Wetland Treating Septic Tank Effluent with Emphasis on Organic and Ammonia Removals. Water. Air. Soil Pollut. 2018, 229, 1–10.
17. Luo, W.; Yang, C.; He, H.; Zeng, G.; Yan, S.; Cheng, Y. Novel Two-Stage Vertical Flow Biofilter System for Efficient Treatment of Decentralized Domestic Wastewater. Ecol. Eng. 2014, 64, 415–423.
18. Latrach, L.; Ouazzani, N.; Masunaga, T.; Hejjaj, A.; Bouhoum, K.; Mahi, M.; Mandi, L. Domestic Wastewater Disinfection by Combined Treatment Using Multi-Soil-Layering System and Sand Filters (MSL-SF): A Laboratory Pilot Study. Ecol. Eng. 2016, 91, 294–301.
19. Wu, J.; Wang, Z. A Hybrid Model for Water Quality Prediction Based on an Artificial Neural Network, Wavelet Transform, and Long Short-Term Memory. Water. 2022, 14, 610.
20. Alnahit, A.O.; Mishra, A.K.; Khan, A.A. Stream Water Quality Prediction Using Boosted Regression Tree and Random Forest Models. Stoch. Environ. Res. Risk Assess. 2022, 36, 2661–2680.

• Introduction section must be written in a more quality way, i.e., more up-to-date references addressed. Please target the specific gap.

To improve the introduction section, other paragraphs have been added, and the references have been updated as indicated in the introductory section on pages 1-2, lines 93-132

• The novelty of the work must be clearly addressed and discussed, compare previous research with existing research findings, and highlight novelty.

As indicated by the reviewer, the originality of the work has been addressed and discussed on page 3, lines 115-132 in the following manner:

Previous studies have mainly focused on the removal of organic matter and nutrients [30–32], whereas recent research has focused more on coliform bacteria and microbial activity [5,7,9,11,33,34]. In this regard, this paper summarized and discussed MSL performances and highlighted the most studied pollutants and those receiving the least attention. In addition, this review tries to describe the pollutant removal mechanisms based on the latest studies, especially with regard to coliform bacteria, which represents one of the shortcomings of the MSL system. In the same sense, both prior and recent studies agree that aeration, hydraulic loading rate (HLR), and clogging are the MSL key parameters [10,12,16,35,36]. Thus, the current paper attempts to examine their relationships in the MSL field using keyword co-occurrence analysis. Regarding MSL application, most prior studies were carried out at the laboratory or pilot scale [6,15,21,37]. Nowadays, researchers have turned to combine MSL with other treatment technologies, like constructed wetlands and trickling filter at full-scale [20,23]. Thus, this paper compares and evaluates their efficacy to determine which hybrid configuration is most effective in removing pollutants. Regarding MSL modeling, this topic is still emerging, and applied nonlinear methods have accurately simulated MSL performance [6,10,16]. Thus, this paper attempts to quantify and discuss the types of applied models, compare their accuracies, and investigate the most targeted parameters.

5. Song, P.; Huang, G.; An, C.; Xin, X.; Zhang, P.; Chen, X.; Ren, S.; Xu, Z.; Yang, X. Exploring the Decentralized Treatment of Sulfamethoxazole-Contained Poultry Wastewater through Vertical-Flow Multi-Soil-Layering Systems in Rural Communities. Water Res. 2021, 188, 116480.
6. Sbahi, S.; Ouazzani, N.; Hejjaj, A.; Mandi, L. Neural Network and Cubist Algorithms to Predict Fecal Coliform Content in Treated Wastewater by Multi-Soil-Layering System for Potential Reuse. J. Environ. Qual. 2021, 50, 144–157.
7. Latrach, L.; Ouazzani, N.; Hejjaj, A.; Mahi, M.; Masunaga, T.; Mandi, L. Two-Stage Vertical Flow Multi-Soil-Layering (MSL) Technology for Efficient Removal of Coliforms and Human Pathogens from Domestic Wastewater in Rural Areas under Arid Climate. Int. J. Hyg. Environ. Health. 2018, 221, 64–80.
8. Zhou, Q.; Sun, H.; Jia, L.; Zhao, L.; Wu, W. Enhanced Pollutant Removal from Rural Non-Point Source Wastewater Using a Two-Stage Multi-Soil-Layering System with Blended Carbon Sources: Insights into Functional Genes, Microbial Community Structure and Metabolic Function. Chemosphere. 2021, 275, 130007.
9. Song, P.; Huang, G.; An, C.; Shen, J.; Zhang, P.; Chen, X.; Shen, J.; Yao, Y.; Zheng, R.; Sun, C. Treatment of Rural Domestic Wastewater Using Multi-Soil-Layering Systems: Performance Evaluation, Factorial Analysis and Numerical Modeling. Sci. Total Environ. 2018, 644, 536–546.
10. Sbahi, S.; Ouazzani, N.; Latrach, L.; Hejjaj, A.; Mandi, L. Predicting the Concentration of Total Coliforms in Treated Rural Domestic Wastewater by Multi-Soil-Layering (MSL) Technology Using Artificial Neural Networks. Ecotoxicol. Environ. Saf. 2020, 204, 111118.
11. An, C.J.; McBean, E.; Huang, G.H.; Yao, Y.; Zhang, P.; Chen, X.J.; Li, Y.P. Multi-Soil-Layering Systems for Wastewater Treatment in Small and Remote Communities. J. Environ. Informatics 2016, 27, 131–144.
12. Aba, R.P.; Mugani, R.; Hejjaj, A.; Brugerolle de Fraissinette, N.; Oudra, B.; Ouazzani, N.; Campos, A.; Vasconcelos, V.; Carvalho, P.N.; Mandi, L. First Report on Cyanotoxin (MC-LR) Removal from Surface Water by Multi-Soil-Layering (MSL) Eco-Technology: Preliminary Results. Water. 2021, 13, 1403.
13. Sbahi, S.; Ouazzani, N.; Hejjaj, A.; Mandi, L. Nitrogen Modeling and Performance of Multi-Soil-Layering (MSL) Bioreactor Treating Domestic Wastewater in Rural Community. J. Water Process Eng. 2021, 44, 102389.
14. Song, P.; Huang, G.; An, C.; Zhang, P.; Chen, X.; Ren, S. Performance Analysis and Life Cycle Greenhouse Gas Emission Assessment of an Integrated Gravitational-Flow Wastewater Treatment System for Rural Areas. Environ. Sci. Pollut. Res. 2019, 26, 25883–25897.
15. Zhang, P.; Huang, G.; An, C.; Fu, H.; Gao, P.; Yao, Y.; Chen, X. An Integrated Gravity-Driven Ecological Bed for Wastewater Treatment in Subtropical Regions: Process Design, Performance Analysis, and Greenhouse Gas Emissions Assessment. J. Clean. Prod. 2019, 212, 1143–1153.
16. Ho, C.C.; Wang, P.H. Efficiency of a Multi-Soil-Layering System on Wastewater Treatment Using Environment-Friendly Filter Materials. Int. J. Environ. Res. Public Health. 2015, 12, 3362–3380.
17. Luanmanee, S.; Attanandana, T.; Masunaga, T.; Wakatsuki, T. The Efficiency of a Multi-Soil-Layering System on Domestic Wastewater Treatment during the Ninth and Tenth Years of Operation. Ecol. Eng. 2001, 18, 185–199.
18. Masunaga, T.; Sato, K.; Mori, J.; Shirahama, M.; Kudo, H.; Wakatsuki, T. Characteristics of Wastewater Treatment Using a Multi-Soil-Layering System in Relation to Wastewater Contamination Levels and Hydraulic Loading Rates. Soil Sci. Plant Nutr. 2007, 53, 215–223.
19. Khalifa, J.; Ouazzani, N.; Hejjaj, A.; Mandi, L. Remediation and Disinfection Capabilities Assessment of Some Local Materials to Be Applied in Multi-Soil-Layering (MSL) Ecotechnology. Desalin. Water Treat. 2020, 178, 53–64.
20. Liu, C.; Huang, G.; Song, P.; An, C.; Zhang, P.; Shen, J.; Ren, S.; Zhao, K.; Huang, W.; Xu, Y. Treatment of Decentralized Low-Strength Livestock Wastewater Using Microcurrent-Assisted Multi-Soil-Layering Systems: Performance Assessment and Microbial Analysis. Chemosphere 2022, 294, 133536.
21. Luanmanee, S.; Boonsook, P.; Attanandana, T.; Wakatsuki, T. Effect of Organic Components and Aeration Regimes on the Efficiency of a Multi-Soil-Layering System for Domestic Wastewater Treatment. Soil Sci. Plant Nutr. 2002, 48, 125–134.
22. Guan, Y.; Zhang, Y.; Zhong, C.N.; Huang, X.F.; Fu, J.; Zhao, D. Effect of Operating Factors on the Contaminants Removal of a Soil Filter: Multi-Soil-Layering System. Environ. Earth Sci. 2015, 74, 2679–2686.
23. Latrach, L.; Ouazzani, N.; Hejjaj, A.; Zouhir, F.; Mahi, M.; Masunaga, T.; Mandi, L. Optimization of Hydraulic Efficiency and Wastewater Treatment Performances Using a New Design of Vertical Flow Multi-Soil-Layering (MSL) Technology. Ecol. Eng. 2018, 117, 140–152.

• Page 1 Line 41 need a reference, cite this article from literature- Role of nanomaterials in the treatment of wastewater: a review.

As suggested by the reviewer, the requested reference was added to the paper on page 2, line 83 as follows.

4. Yaqoob, A.A.; Parveen, T.; Umar, K.; Mohamad Ibrahim, M.N. Role of Nanomaterials in the Treatment of Wastewater: A Review. Water 2020, 12, 495.

• Page 2 line 49 need a reference. Please cite this- Introduction of adsorption techniques for heavy metals remediation.  

The proposed reference was included in the work on page 2 line 93 as recommended by the reviewer as follows:

8. Idris, M.O.; Yaqoob, A.A.; Ibrahim, M.N.M.; Ahmad, A.; Alshammari, M.B. Introduction of Adsorption Techniques for Heavy Metals Remediation. In Emerging Techniques for Treatment of Toxic Metals from Wastewater; Elsevier, 2023; pp. 1–18.

• The last paragraph of introduction seems very weird, please do not use long sentence.

Short sentences have been used to rewrite the final paragraph of the introductory section, as seen on page 3, lines 143-153.

Based on the literature and limitations, the main focus of this bibliometric and review paper is to find out the effective working of multi-soil-layering eco-technology, considering the key efficiency parameters, modeling approaches, and key knowledge gaps.

The main objectives of this paper are :

• to conduct a bibliometric analysis in the field of MSL technology;
• to provide a comprehensive summary of MSL aspects (e.g., performance, removal mechanisms, etc.);
• to issue a comparative profile of the MSL with other eco-friendly technologies;
• to evaluate models applied to simulate MSL performance;
• to highlight the MSL challenges and provide a road map for future research.

• What is the main challenge? Please highlight this in the introduction part.

The MSL main challenge has been mentioned in the introductory section, as suggested by the reviewer, as seen on page 2 lines 92-98 as follows:

Although recent studies have been concerned with the bacterial community in the MSL system and its sanitary efficiency [5,6,9–11]. Thus, some concerns about the MSL’s efficiency are still unclear [12,13]. For instance, the removal of fecal indicator bacteria has moved from 1.01 to 1.28 log units using a single-stage MSL system [7,14]. However, given the necessity to reuse the MSL effluent in agriculture, this percentage still falls short of the ideal level [6]. Therefore, the vital challenge with MSL is treating fecal indicator bacteria, whose level in treated water is a sign of good sanitary efficiency.

5. Song, P.; Huang, G.; An, C.; Xin, X.; Zhang, P.; Chen, X.; Ren, S.; Xu, Z.; Yang, X. Exploring the Decentralized Treatment of Sulfamethoxazole-Contained Poultry Wastewater through Vertical-Flow Multi-Soil-Layering Systems in Rural Communities. Water Res. 2021, 188, 116480.
6. Sbahi, S.; Ouazzani, N.; Hejjaj, A.; Mandi, L. Neural Network and Cubist Algorithms to Predict Fecal Coliform Content in Treated Wastewater by Multi-Soil-Layering System for Potential Reuse. J. Environ. Qual. 2021, 50, 144–157.
7. Latrach, L.; Ouazzani, N.; Hejjaj, A.; Mahi, M.; Masunaga, T.; Mandi, L. Two-Stage Vertical Flow Multi-Soil-Layering (MSL) Technology for Efficient Removal of Coliforms and Human Pathogens from Domestic Wastewater in Rural Areas under Arid Climate. Int. J. Hyg. Environ. Health. 2018, 221, 64–80.
8. Zhou, Q.; Sun, H.; Jia, L.; Zhao, L.; Wu, W. Enhanced Pollutant Removal from Rural Non-Point Source Wastewater Using a Two-Stage Multi-Soil-Layering System with Blended Carbon Sources: Insights into Functional Genes, Microbial Community Structure and Metabolic Function. Chemosphere. 2021, 275, 130007.
9. Song, P.; Huang, G.; An, C.; Shen, J.; Zhang, P.; Chen, X.; Shen, J.; Yao, Y.; Zheng, R.; Sun, C. Treatment of Rural Domestic Wastewater Using Multi-Soil-Layering Systems: Performance Evaluation, Factorial Analysis and Numerical Modeling. Sci. Total Environ. 2018, 644, 536–546.
10. Sbahi, S.; Ouazzani, N.; Latrach, L.; Hejjaj, A.; Mandi, L. Predicting the Concentration of Total Coliforms in Treated Rural Domestic Wastewater by Multi-Soil-Layering (MSL) Technology Using Artificial Neural Networks. Ecotoxicol. Environ. Saf. 2020, 204, 111118.

• The main objective of the work must be written in the clearer and more concise way at the end of the introduction section.

As the reviewer pointed out, the primary objective of this paper has been reformulated in a clear and concise manner at the end of the introduction section as indicated on page 3, lines 143-153:

Based on the literature and limitations, the main focus of this bibliometric and review paper is to find out the effective working of multi-soil-layering eco-technology, considering the key efficiency parameters, modeling approaches, and key knowledge gaps. The main objectives of this paper are :

• to conduct a bibliometric analysis in the field of MSL technology;
• to provide a comprehensive summary of MSL aspects (e.g., performance, removal mechanisms, etc.);
• to issue a comparative profile of the MSL with other eco-friendly technologies;
• to evaluate models applied to simulate MSL performance;
• to highlight the MSL challenges and provide a road map for future research.
• Please provide space between numbers and units. Please revise your paper accordingly since some issue occurs in several spots in the paper. 

The spacing between numbers and units has been changed across the whole document at the reviewer's suggestion.

• Please check the abbreviations of words throughout the article. All should be consistent.

In accordance with the reviewer's recommendation, we have checked and reviewed all the word abbreviations used in the article.

• Please add a comparative profile section as it is a review.

According to the reviewer's notes, lines 687-706 of the text have been inserted as a comparison section as follows:

3.6. Multi-soil-layering and alternative treatment technologies

This paper also aimed to investigate potential alternative wastewater technologies for MSL through a comprehensive literature analysis. The findings reveal that constructed wetland, lagoon, and sand filter technologies are widely used as rural sewage treatment technologies. Table 2 provides a summary of the comparison's findings. These alternative methods can be used to treat wastewater in rural regions, according to preliminary comparisons. Additionally, they have the trait of being low-cost and eco-friendly technologies. Thus, the techno-economic evaluation shows that MSL is a suitable option since it can withstand high organic loading rates and only require a small area for operation. Additionally, MSL can withstand the negative effects of odor or insects. However, MSL’s sanitary efficiency falls short of expectations. Additionally, MSL is an emerging technology and shows a sensitivity to clogging at high HLR. Therefore, conducting more studies in this field has become an urgent necessity to improve MSL's performance.

Table 2. Main advantages and disadvantages of various rural sewage treatment technologies

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The updated version checked....done for all my comments. It can be accepted and published as it is.

Reviewer 2 Report

Accepted in the present form